Aromatic polyimides are normally prepared from an aromatic tetracarboxylic dianhydride and an aromatic primary diamine. When these materials are reacted at relatively low temperatures in a suitable solvent, typically a dipolar aprotic solvent such as N-methylpyrrolidone or N,N-dimethylacetamide, an aromatic polyamic acid is formed, usually as a viscous solution sometimes referred to as a varnish. When heated to a temperature above about 140.degree. C. imidization occurs such that a polyimide polymer is formed.
It is known from Japan Kokai 57-200452 and Japan Kokai 57-200453 that finely divided aromatic polyimides from a variety of aromatic tetracarboxylic dianhydride and aromatic diamines can be formed by rapidly heating solutions of polyamic acid in a polar organic solvent such as N-methylpyrrolidone, N,N-dimethylformamide, etc. to 160.degree.-300.degree. C. In this way, polyimide powders suitable for use in compression molding are formed from solutions of polymers such as 3,3',4,4'-biphenyltetracarboxylic dianhydride 4,4'-diaminodiphenyl ether polymer, pyromellitic dianhydride 4,4'-diaminodiphenyl ether polymer, and 3,3',4,4'-biphenyltetracarboxylic dianhydride 4,4'-diamininophenylmethane polymer.
In U.S. Pat. No. 3,708,459 the preparation of polyimide molding powders having surface areas between 100 and 900 square meters per gram is disclosed. These powders are obtained from a polymerizate of a polyamic acid prepolymer and an imide prepolymer. The prepolymers result from the reaction of at least one polyfunctional amine, one or more polyfunctional anhydride and nadic anhydride unsubstituted or substituted with lower alkyl. The prepolymer is thermally polymerized to obtain the desired product.
Polyimide polymers based on use of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane as the sole or predominant aromatic primary diamine are of considerable interest because of their desirable high temperature properties. However, the production of such polymers in powder form presents a number of problems. Certain prior methods for converting the polyamic acid to the corresponding polyimide yield the polyimide in the form of solids which require extensive grinding. However, substantial portions of the product may not even be amenable to grinding. Other prior methods are fraught with difficulties caused by the tendency of the wet polyimide polymer to agglomerate into a stringy or tacky mass which can foul reactor and agitator surfaces. Further, such tacky masses cannot be removed from the reactor in any commercially practical manner. Moreover, the solvent tends to remain occluded in such swollen, tacky mass.
A desirable contribution to the art would be a process in which such difficulties may be eliminated, or at least greatly reduced.